I think you did not cycle the tank. Check our articles section and read up on the cycling process anywhere on the net. In the process, a dead decaying organism(s) creates an ammonia source and kicks off the cycling process.
I can't see how your fish shop knew that happened if you simply set it up and "let it run for a month". If I'm right, everything in your tank could die.
"Life may not be the party we hoped for but while we are here we might as well dance!"